This invention relates to a method and apparatus for producing X-rays of selectable wavelengths and intensities and to a system utilizing such X-rays to detect specific chemical elements in a sample by analysis of X-ray fluorescence therefrom.
When exposed to X-rays, most substances will fluoresce X-rays that have a specific wavelength or combination of specific wavelengths that are characteristic of the chemical elements in the substance. The wavelength spectrum of the X-ray fluorescence is not determined by that of the incident X-rays which are causing the fluorescence but is instead determined by the elemental composition of the substance. As the fluorescent X-ray spectrum is distinct for each different element, this offers a technique for rapidly and nondestructively detecting the presence of a specific element in a sample or for identifying the total chemical composition of the sample. The complete fluorescent X-ray spectrum of an unknown substance provides a fingerprint for that substance which contains all the information needed to identify and quantitatively analyze most of the elements that may be present in the substance.
This technique has heretofore been used only on a very limited basis, mostly within the confines of research laboratories, to analyze specific substances for specific elements using an assembly of devices useful only for that particular analysis. One major problem is that conventional X-ray tubes or sources are not easily controllable with respect to the wavelength spectrum which is produced. The conventional X-ray tube consists of means for accelerating a beam of electrons toward a target or anode generally composed of a refractory metal such as tungsten or the like. The resultant X-rays have two principal components. One consists of the characteristic X-ray fluorescence wavelengths of the target material and the other is a polychromatic spectrum known as bremmstrahlung which contains a more or less continuous distribution of wavelengths. The characteristic specific wavelength of the target material is unchangeable and is accompanied by a heterogeneous mixture of many wavelengths. Use of such an X-ray tube complicates the X-ray fluorescence analysis technique described above and greatly limits the accuracy of the result. While, as pointed out above, the wavelength of X-ray fluorescence from a sample is not determined by the wavelength of the incident X-rays, this does not mean that control of the wavelength of the incident X-rays is unimportant. First, the intensity of the X-ray fluorescence from the sample is strongly dependent upon the wavelength of the incident X-rays. Second, the presence of the bremmstrahlung spectrum in the vicinity of the sample can greatly interfere with detection of the desired sample fluorescence.
Accordingly, efficient use of the X-ray fluorescence analysis technique requires a substantially bremmstrahlung-free source of X-rays having a specific wavelength spectrum. Further, if the technique is to be used to analyze a sample for a variety of initially unknown possible chemical element constituents, the source of X-rays should be readily adjustable to produce any selected one of a large number of specific wavelengths or wavelength spectra since it may be necessary to sequentially irradiate the sample with a series of different wavelengths to produce a strong fluorescence response from each of the various constituent elements. This change of output wavelengths should be accomplishable quickly and by a simple adjustment of controls either manually or preferably by an automatic feedback system which responds to the sample X-rays fluorescence. These objectives cannot be accomplished if the X-ray source apparatus must be disassembled and reconstructed in part to produce a desired specific wavelength spectra.
Moreover, an X-ray source producing selectable wavelengths and intensities can offer important advantages in contexts other than the spectrometric analysis of substances as discussed above. In radiology as used for medical purposes or for the inspection of industrial parts, for example, the image which is obtained of the internal structure of a subject is useful only to the extent that different internal elements of the subject are distinguishable in the image. The contrast in such an image, between different internal areas of the subject, is again strongly influenced by the wavelength of the incident X-rays. The particular wavelength which provides the best contrast in an image of a medical patient between bone structure and the surrounding soft tissue may not be the same wavelength which provides the best contrast in distinguishing tumorous tissue from adjacent normal tissue. Thus, the versatility of radiology equipment of this kind may also be greatly enhanced by an X-ray source providing for selection of any of a plurality of different wavelength spectra.